1. Field of the Invention
This invention relates generally to a plane type multi-digit indicating apparatus for use in small electronic appliances such as small electronic calculators, electronic clocks of a digital display type and the like, and more particularly to improvements in envelopes or enclosures for such appliances.
2. Description of the Prior Art
There are various conventional structures for multi-digit indicator tubes for use in electronic calculators or other electronic appliances. It is remarkable that the electronic calculators have recently come into wide use and the popularity of the electronic calculators is still more increasing. The electronic calculators which are now increasing in demand are smaller in size, easy to carry and less expensive. Accordingly, there are great demands for the indicator tubes suitable for use in these electronic calculators.
In order to meet the recent demands for the small-sized electronic calculators, flat-type indicator tubes have been proposed. One of the indicator tubes of this type includes a flat rectangular base plate and a cover plate having a transparent slightly curved front viewing window and a peripheral edge projecting horizontally around the cover plate. The entire configuration of the cover plate is generally a flat-bottomed boat shape, and the cover plate is secured to the base plate at the peripheral edge thereof to thereby form an envelope.
The indicator tube of this type has been widely used, because it is thin and handy. Also, the cover plate is formed into a flat-bottomed boat shape having the slightly curved front surface, which make it possible to form the envelope an appealing streamlined shape. In addition, this type of the indicator tube does not require a spacer which is placed between the base plate and cover plate when these plates are joined together, and the sealing of the base plate and cover plate are relatively easy, because the cover plate is directly secured to the base plate at the peripheral edge thereof.
However, the cover plate according to this type of the indicator tube is usually made of transparent glass, and must be formed in the flat-bottomed boat shape one by one by heat processing so that distortion or shrinkage which is deleterious to observe activated indicator elements within the envelope may not be created on the upper viewing window. Accordingly, the cost of the indicator tube using the cover plate described above becomes high. Also, it is somewhat difficult to fabricate the upper viewing window in a perfect flat condition so as to be easy to observe the indicator elements within the envelope, because melted glass tends to accumulate thickly at the four corners of the cover plate when the cover plate is manufactured. Further, there is a limitation with respect to the width of peripheral edge of the cover plate, even if the size of the indicator tube is reduced. This is considered to be a bottleneck for making the indicator tube compact.
The conventional indicator tube of another type includes a flat rectangular base plate and a flat face plate which is hermetically sealed to the base plate at the peripheral edges thereof by means of a spacer disposed therebetween so that a chamber may be provided between the base plate and the face plate.
The indicator tube of this type has been widely used as well. The face plate used in this indicator tube is prepared by simply cutting a flat glass plate into a rectangular shape which is easily available on the market at low price. Accordingly, the envelope for enclosing the indicator elements can be relatively easy to fabricate. Also, the indicator elements within a casing can be easy to observe, because the flat glass plate is used as the face plate. However, this type of indicator tube must use the spacer, as for example, baked or temporary baked ceramic spacer formed in a shape of hollow frame or a glass spacer formed in a shape of hollow frame by etching a glass plate. Thus, the cost of the spacers is fairly expensive. In addition, the envelope of the indicator tube of this type includes sharp peripheral edges as they cut the glass plate around the base plate and face plate, which makes the outward appearance of the indicator tube unshapely and less appealing. Also, there is a limitation with respect to the thickness of the spacer frame in this type of indicator tube even if it is reduced in size. This is considered to be a problem for making the indicator tube compact.